1. Field of the Invention
The present invention relates to electrographic development apparatus and process which provide enhanced development with partially-conductive developer.
2. Description of the Prior Art
In the art of electrographic reproduction, e.g., electrophotographic copying, one direction of advance is toward the production of higher quality copies at faster copying rates, e.g., to serve the intermediate run duplication market which presently requires offset printing techniques. Rapid, high quality electrographic development, i.e., the application of toner to a latent electrostatic image, has presented particular problems to efforts in this direction. This is particularly true with respect to development of large solid or large continuous tone areas of the image (i.e., image areas which bear a generally uniform charge corresponding to a generally uniform shade on the original).
Present commercial apparatus have predominately utilized cascade or magnetic brush systems, incorporating a development electrode in some form to enhance solid area development. The function of the development electrode in these devices is to cause the electric field of the large solid area to be external of the image member (rather than within the member) and thus to more accurately reflect the electrostatic surface charge of the area. This is accomplished in cascade systems by locating an electrically conductive plate opposite the development zone; the plate can be biased to a potential level to control background development. In magnetic brush systems, the metallic cylinder of the brush can be similarly biased to perform as a development electrode.
Numerous variations of such systems have been devised to facilitate increased density and uniformity of large solid areas in high speed development. For example, extended development zones and automatic bias variation (in response to feedback from the electrostatic image) have been utilized to increase density and improve uniformity. However, even with the best of these systems, there remain significant aspects for improvement.
Recently issued U.S. Pat. No. 4,076,857 discloses a new development approach which offers advantage in the attainment of increased image density in high speed operation. This approach, in general, utilizes a partially-conductive developer, as distinguished from most prior art developer mixtures, which can be characterized as substantially insulative. In this new approach, the combination of using such partially-conductive developers and of applying the developer in controlled conditions which cause an "electrical breakdown" of the developer mixture between the applicator and the image member causes a remarkable increase in the extent of development, i.e., the quantity of toner transferred to the image member.
We have observed that, although partially-conductive developer mixtures offer advantages in the breakdown development mode and other modes of development, certain non-uniformities exist in the development of large solid image areas with such mixtures. Specifically we have noted that certain portions (particularly leading and trailing edge portions) of large solid image areas are developed disproportionately in density (either much too light or much too dark). These non-uniform development effects can, in some instances, detract significantly from the overall image quality.